Radar detectors have been known for some considerable time. Police radar devices that are used in the detection and measuring of the speed of moving vehicles, and also intrusion and zone security devices that are used in buildings, operate in two designated frequency bands. They are the X band, at 10.525 GHz, and the K band at 24.150 GHz. Presently, all radar operating devices intended for speed detection and building intrusion or zone security purposes operate at one or the other of those frequencies. Moreover, because of the increased use of building and zone security devices, and the tendency of police radar to be used in pulsed manner--that is, only infrequently and only when the operator believes he has a speeding vehicle within his range --it has become more necessary to provide radar detectors that not only detect a true signal and give an appropriate warning, it is also desirable that radar detectors should not give false signals.
One way of overcoming an excessive number of false signals is to decrease the sensitivity of a radar detector; but that creates the problem that an operating radar may not be detected until the vehicle carrying the detector is within the accurate measuring range of the radar device. Generally, radar devices such as police radar give off a field at their operating frequency that extends for a much greater range than the range at which they will give accurate and meaningful vehicle speed information; and it is those low level and long range signals that are required to be picked up. At the same time, it is desirable that the detector should not give off an alarm each time that a radar device in a building for intruder detection or zone security is detected, due to its radiated field.
Thus, it becomes desirable to provide a radar detector that will automatically adjust its sensitivity; and moreover it is desirable to provide a radar detector that will give an indication as to the signal strength of a signal that is causing an alarm. Still further, it is desirable that the radar detector should give an almost instantaneous alarm in the event of a high level signal that can be identified as being from such as a police radar unit, as opposed to a high level signal that might be detected as a moving vehicle drives past a building having a zone security radar system with high level radiation.
The prior art includes REID et al U.S. Pat. No. 4,157,550, issued June 5, 1979. That patent teaches a microwave detector that operates at both of the X-band and K-band frequencies, but is particularly directed to the mounting of the detection diodes with respect to the horn.
SCHWEITZER U.S. Pat. No. 4,196,393 issued Apr. 1, 1980 teaches a further X-band or K-band operating microwave signal detector, and that patent also is mroe related to the mounting of the detection devices, with little attention to signal handling.
Yet a further radar signal detector is taught in MOSHER U.S. Pat. No. 4,315,261, issued Feb. 9, 1982. Mosher mixes received radar signals with a swept frequency signal, that is then passed to a narrow band FM discriminator, but that is a frequency compression discriminator.
In contradistinction to the prior art, the present invention is particularly directed to the provision of signal processing circuits, by which false alarm conditions may be substantially avoided.
Indeed, the actual detection and front end of a radar detection device is beyond the scope of the present invention, but specific examples are given by way of illustration. A novel horn construction which is particularly adapted for use with radar detection and signal processing circuits according to the present invention is taught in U.S. Pat. No. 4,571,593 issued Feb. 18, 1986, in the name of the present inventor and assigned to a common Assignee, and filed of even date herewith.
It can be assumed, for example, that circuits according to the present invention are particularly adapted to operate at 10.525 GHz (the X-band) or 24.150 GHz (the K-band); and that signals at either of those frequencies will be mixed with a local oscillator signal. In any event, by properly mixing an RF signal at either frequency with a local oscillator signal or a harmonic thereof, an intermediate frequency of 1.033 GHz may be obtained. How that intermediate frequency signal--which only exists in the presence of a detected radar or microwave signal at either of the X-band or K-band frequencies--is processed, and how that signal may be measured as to its strength, and how the sensitivity of processing may be adjusted according to background noise conditions, are features of the present invention that are described in greater detail hereafter.
What the present invention provides is a radar detection and signal processing circuit where, once an intermediate frequency signal from a first mixer is obtained, it is fed to a second mixer together with a signal with a swept frequency oscillator, with the output from that second mixer being fed to a band pass filter. Thus, in the presence of a microwave frequency signal that has been converted to the first intermediate frequency, there is a pair of signals that are spaced from the band pass filter centre frequency. Both signals have a relatively narrow (400 KHz) band width; and each is a burst of signals in a substantially sinusoidal envelope. The first occurs when the output from the varactor tuned oscillator 24 is higher than the IF signal by an amount equal to the band pass filter centre frequency (10 MHz), and the second occurs when the output from the varactor tuned oscillator is lower than the IF signal frequency by the same amount. Those signals may then be processed as to their incidence at either low or high thresholds, on successive sweeps, and as to the incidence of signals above the low threshold level that may be distinguished from noise; whereby alarm signals may be given substantially only in the presence of a detected radar signal from such as a police radar, in the X-band or K-band frequencies (or such other frequency as may be assigned in the future, and for which appropriate detection and mixing circuitry may then be readily devised). [Of course, in this invention, as briefly discussed above and described in detail hereafter, it will be appreciated that reference to the second miser being fed by a signal from a swept frequency oscillator could also be with respect to the first mixer; that is, the first mixer could be fed with a signal from a swept frequency oscillator while the second mixer is fed with a signal from a fixed frequency low oscillator, together with the data signal being processed.